Project summary: Imaging biological tissues is an important part of identifying disease. Among all the imaging methods, immunostaining in which a specific biomarker is combined with a bright imaging fluorophore is most reliable. Near-infrared (NIR) emitting materials would be ideal because they can avoid the interference of tissue autofluorescence and have minimal absorbance by water. Currently the only FDA approved NIR dye is indocyanine green (ICG) fluorophore which has been explored extensively to detect residual cancer cells in the surgical wound through tumors' enhanced permeability and retention (EPR) effect. However, ICG is not bright with a low quantum yield (QY) of about 2% in water. Recently halide perovskites nanocrystals (HPNC) of formula ABX3 (where A=organic ammonium cation or Cs, B=metal cation such as Pb or Sn, and X=halide anion such as Cl, Br, or I) have been shown to be an outstanding new photoluminescent material with a high QY of nearly 100%. We propose to use HPNC as a bright fluorophore to create NIR molecular probe that can image tissues at a single-cell level without the need of microscopic amplification. It is anticipated that the success of such a study will lead to a bright molecular probe that can be used in the clinical setting for a variety of imaging applications. For example, it can be used to determine whether a surgery has removed all the cancer cells from patients during surgery by assessing the tumor margins. The challenge is that the NIR HPNCs are unstable in ambient condition and have to be protected from moisture, let alone immersed in water for biological applications. Recently it was shown that silica-coated cesium lead halide particles are stable and can be used for imaging cells through internalization. For bioimaging, stable aqueous suspensions are needed for biomolecule conjugation process. However, so far there has been no method to generate stable aqueous halide perovskite particle suspensions. In preliminary studies, we showed that using a hydrophobic-hydrophilic block-copolymer, it is possible to create aqueous suspensions of MAPbBr3. However, the suspensions could exist for only 30 minutes and settled out afterward due to the insufficient protection from water damage to the particles. More recently we used silica precursor to coat and protect the MAPbBr3 NCs first followed by the addition of a hydrophilic- hydrophobic-hydrophilic triblock copolymer to disperse the particles, we could produce an aqueous suspension of MAPbBr3 that is stable for a month. The aim of the research is to create stable aqueous NIR HPNC suspensions of MAPbI3. In task 1, we will use silica to coat the MAPbI3 first followed by the addition of triblock copolymers and subsequently transferring the solvent from toluene to water. In task 2 we will verify the process by attaching antibodies to the HPNC and use them to stain cells. A good signal-to-noise ratio of >10 would indicate a reasonable staining.